In wireless communication systems, the characteristics of a transmission path change. Factors involved in a change of the characteristics of a transmission path include movement of a mobile stations and the effect of rain. In particular, radio waves in high frequency bands, such as microwaves and millimeter waves, are increasingly attenuated due to rain. Hence, outdoor wireless communication using radio waves in a high frequency band poses a difficulty in maintaining the communication distance in rain.
To address this problem, Patent Literature 1 discloses a wireless communication system that changes modulation systems according to the reception power. Specifically, a measured reception power is compared to the reception power necessary for the modulation system currently in use (required reception power), and, if the measured reception power is larger than the required reception power by a value not less than a predetermined value, the modulation system is changed to a system having a larger modulation multi-level number than that of the modulation system currently in use. If the measured reception power is smaller than the required reception power by a value not less than a predetermined value, the modulation system is changed to a system having a smaller modulation multi-level number than that of the modulation system currently in use. A modulation system having a smaller modulation multi-level number typically requires a smaller reception power. Thus, when the reception power is reduced due to rain, the modulation system is switched to a system requiring a smaller reception power, that is, having a smaller modulation multi-level number, so as to increase the communication distance in the rain.